Emulsion graft polymer

ABSTRACT

Emulsion graft copolymers are composed of 
     A) from 5 to 18% by weight of a first stage comprising 
     a 1 ) from 85 to 99% by weight of methyl methacrylate, 
     a 2 ) from 1 to 15% by weight of C 1  -C 8  -alkyl acrylate, 
     a 3 ) from 0 to 2% by weight of allyl methacrylate and 
     a 4 ) from 0 to 3% by weight of further di- or polyfunctional comonomers, 
     B) from 25 to 35% by weight of a second stage comprising 
     b 1 ) from 10 to 25% by weight of vinylaromatic monomers, 
     b 2 ) from 75 to 90% by weight of C 1  -C 20  -alkyl acrylate and 
     b 3 ) from 0 to 3% by weight of allyl methacrylate, 
     C) from 30 to 40% by weight of a third stage comprising 
     c 1 ) from 15 to 27% by weight of vinylaromatic monomers, 
     c 2 ) from 73 to 85% by weight of C 1  -C 20  -alkyl acrylate and 
     c 3 ) from 0 to 3% by weight of allyl methacrylate and 
     D) from 15 to 30% by weight of a fourth stage comprising 
     d 1 ) from 85 to 96% by weight of methyl methacrylate, 
     d 2 ) from 3.8 to 10% by weight of C 1  -C 8  -alkyl acrylate, 
     d 3 ) from 0 to 2% by weight of further di- or polyfunctional comonomers and 
     d 4 ) from 0.2 to 3% by weight of a regulator, 
     with the proviso that the ratio of the amounts of vinylaromatic monomers c 1  :b 1  is from 1.3:1 to 1.9:1 and the sum of the components A to D is 100% by weight.

The present invention relates to emulsion graft copolymers which arecomposed of

A) from 5 to 18% by weight of a first stage comprising

a₁) from 85 to 99% by weight of methyl methacrylate,

a₂) from 1 to 15% by weight of C₁ -C₈ -alkyl acrylate,

a₃) from 0 to 2% by weight of allyl methacrylate and

a₄) from 0 to 3% by weight of further di- or polyfunctional comonomers,

B) from 25 to 35% by weight of a second stage comprising

b₁) from 10 to 25% by weight of vinylaromatic monomers,

b₂) from 75 to 90% by weight of C₁ -C₂₀ -alkyl acrylate and

b₃) from 0 to 3% by weight of allyl methacrylate,

C) from 30 to 40% by weight of a third stage comprising

c₁) from 15 to 27% by weight of vinylaromatic monomers,

c₂) from 73 to 85% by weight of C₁ -C₂₀ -alkyl acrylate and

c₃) from 0 to 3% by weight of allyl methacrylate and

D) from 15 to 30% by weight of a fourth stage comprising

d₁) from 85 to 96% by weight of methyl methacrylate,

d₂) from 3.8 to 10% by weight of C₁ -C₈ -alkyl acrylate,

d₃) from 0 to 2% by weight of further di- or polyfunctional comonomersand

d₄) from 0.2 to 3% by weight of a regulator,

with the proviso that the ratio of the amounts of vinylaromatic monomersc₁ :b₁ is from 1.3:1 to 1.9:1 and the sum of the components A to D is100% by weight.

The present invention furthermore relates to a process for thepreparation of these emulsion graft copolymers and to the use thereof.

Emulsion graft copolymers based on vinyl monomers are generally knownand are used by themselves but in particular as additives in plastics,such as acrylate- and methacrylate-based homo- or copolymers. Theiraddition results in an improvement in the impact strength of suchacrylate resins, which can be processed, for example, to moldings,sheets, films and organic glass (acrylic glass). The other importantbasic properties of acrylate resins, such as high light transmittanceand clarity, weather resistance, heat distortion resistance andmechanical strength, may however be adversely affected by theseadditives. It is therefore generally desirable to keep the undesirableproperty changes as small as possible or--if possible--to prevent themand at the same time to achieve an increase in the impact strength bymeans of emulsion graft copolymers having a multistage structure andspecial composition.

DE-A 22 53 689 discloses multistage emulsion graft copolymers. Thepolymers described in detail there are composed of three stages, ie. acore and a first and a second graft shell, the core and second shellconsisting of a hard, nonelastomeric polymer and the first shell of anelastomeric polymer. Furthermore, this publication points out thepossibility of a multistage structure of the graft polymers comprisingalternating hard and elastomeric phases.

Moreover, EP-A-512 333 describes emulsion graft copolymers which arecomposed of a hard core and four graft shells, hard and soft stagesalternating.

If the known emulsion graft copolymers are used as additives inthermoplastics, in particular acrylate resins, such as polymethylmethacrylate, their impact strength increases but edges of moldings orcoatings on moldings have a yellowish tinge, which is undesirable. Inaddition, the known emulsion graft copolymers cannot be sufficientlythoroughly dispersed in the thermoplastics so that the haze will besatisfactorily low for many applications.

A further problem is the formation of shell-like indentations inextruded sheets, in particular of polymethyl methacrylate toughened withthe known emulsion graft copolymers. The formation of shell-likeindentations is understood as meaning shell-like indentations which format the edges when the sheets are sawn for processing. In addition,injection molded parts splinter or crack when sprues are mechanicallyremoved.

It is an object of the present invention to provide emulsion graftcopolymers as impact strength improvers for thermoplastic materials,which impart high impact strength to these materials and at the sametime improve the mechanical and optical properties, such as yellow tingeof edges and haze, and reduce the tendency to form shell-likeindentations.

We have found that this object is achieved by the emulsion graftcopolymers defined at the outset. Preferred embodiments are evident fromthe description.

We have also found a process for their preparation, their use asadditives in thermoplastic materials and moldings, films and coatingscomprising these thermoplastic materials.

The novel emulsion graft copolymers have a four-stage compositioncomprising a hard, nonelastomeric stage (A), an elastomeric, secondstage (B), an elastomeric, third stage (C) and finally an outer, hard,nonelastomeric, fourth stage (D), the individual stages being present inthe following amounts:

from 5 to 18, preferably from 10 to 16, % by weight of a first stage(A),

from 25 to 35, preferably from 28 to 33, % by weight of a second stage(B),

from 30 to 40, preferably from 32 to 36, % by weight of a third stage(C) and

from 15 to 30, preferably from 18 to 26, % by weight of a fourth stage(D),

with the proviso that the stages A to D give 100% by weight.

The first stage A of the emulsion graft copolymer consists of

from 85 to 99, preferably from 90 to 98, % by weight of methylmethacrylate (a₁),

from 1 to 15, preferably from 3 to 10, % by weight of C₁ -C₈ -alkylacrylate (a₂),

from 0 to 2, preferably from 0.2 to 1.2, % by weight of allylmethacrylate (a₃) and

from 0 to 3, preferably from 0.1 to 0.5, % by weight of further di- orpolyfunctional comonomers (a₄).

The sum of the components a₁ to a₄ is 100% by weight.

Suitable C₁ -C₈ -alkyl acrylates a₂ are both linear and branched alkylesters of acrylic acid. The preferred C₁ -C₈ -alkyl acrylates includemethyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate,n-butyl acrylate, isobutyl acrylate and tert-butyl acrylate. Amongthese, methyl acrylate and ethyl acrylate are particularly preferred.Mixtures of different C₁ -C₈ -alkyl acrylates may also be used.

Examples of di- or polyfunctional comonomers (a₄) are

alkylene glycol diacrylates, such as ethylene glycol diacrylate,propylene glycol diacrylate, 1,3-butylene glycol diacrylate and1,4-butylene glycol diacrylate,

alkylene glycol dimethacrylates, such as ethylene glycol dimethacrylate,propylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate and1,4-butylene glycol dimethacrylate,

acrylates and methacrylates of glycerol, trimethylolpropane,pentaerythritol, inositol and similar sugar alcohols,

acrylamides and methacrylamides of ethylenediamine and other aliphaticdi- and polyamines,

triacrylamides and trimethacrylamides,

triallyl cyanurate and triallyl isocyanurate and

vinylbenzenes, such as divinylbenzene and trivinyl benzene.

Mixtures of different comonomers a₄ may also be used.

The comonomers (a₄) serve as crosslinking agents. Owing to their abilityto be uniformly incorporated in the polymeric phase, they are alsoreferred to as polymerization crosslinking agents. During thepolymerization reaction, the rate at which they are consumed isessentially comparable to that of the main monomer, resulting incrosslinking within the phase.

The comonomers (a₄) may be present in an amount of up to 3% by weight inthe first stage but are preferably not present or present only inamounts of from 0.1 to 0.5% by weight.

Allyl methacrylate (a₃) is present as a graft-linking comonomer in anamount of from 0 to 2% by weight in the first stage, the amountdepending on the desired size of the first stage. In the case of apreferred size of from 50 to 200 nm, the amount of allyl methacrylate isfrom 0.2 to 1.5% by weight.

Graft-linking comonomers ensure external crosslinking and effect, forexample, the linking of the first stage (core phase) to the subsequentpolymerization stage (graft shell) in the synthesis of an emulsion graftcopolymer. Graft-linking agents, such as allyl methacrylate, exhibitthis behavior because a polymerizable double bond (double bond of theacid moiety) polymerizes at a rate comparable to that of the mainmonomer (methyl methacrylate). On the other hand, the double bond of theallyl group reacts at a substantially lower polymerization rate so thatsome of these double bonds are retained unchanged in the core phase atthe end of the polymerization, permitting graft-linking between twophases.

The first stage A has in general a glass transition temperature of morethan 50° C., preferably from 80° to 130° C.

In a second stage B, a monomer mixture comprising

from 10 to 25, preferably from 10 to 20, % by weight of a vinylaromaticmonomer (b₁),

from 75 to 90, preferably from 78 to 88, % by weight of a C₁ -C₂₀ -alkylacrylate (b₂) and

from 0 to 3, preferably from 0.5 to 2, % by weight of allyl methacrylate(b₃)

is polymerized in the presence of the emulsion polymer of the firststage, which is also referred to as a core latex or seed latex. The sumof the components b₁ to b₃ is 100% by weight.

Suitable vinylaromatic monomers (b₁) are styrene, α-methylstyrene,tert-butylstyrene, monochlorostyrene, vinyltoluene and phenyl acrylateand methacrylate.

The vinylaromatic monomers (b₁), in particular styrene, are particularlyimportant since they increase the refractive index. In this way, it canbe matched with the refractive indices of the first stage and of thesubsequent stages. Moreover, the refractive index can also be matched tothat of the thermoplatic material by means of the vinylaromatic monomersof the emulsion graft copolymer.

Among the C₁ -C₂₀ -alkyl acrylates (b₂), the C₂ -C₁₀ -alkyl esters arepreferred. Both straight-chain and branched alkyl esters of acrylic acidmay be used. n-Butyl acrylate and 2-ethylhexyl acrylate are particularlypreferred. Mixtures of different acrylates (b₂) may also be used.

Furthermore, from 0 to 3, preferably from 0.5 to 2, % by weight of thegraft-linking agent allyl methacrylate (b₃) are present in the monomermixture of the second stage.

The glass transition temperature of stage B is in general less than 0°C., preferably from -10° to -40° C.

The third stage is elastomeric like the second stage and consists of

from 15 to 27, preferably from 17 to 25, % by weight of vinylaromaticmonomers c₁,

from 73 to 85, preferably from 73 to 80, % by weight of C₁ -C₂₀ -alkylacrylate c₂ and

from 0 to 3, preferably from 0.5 to 2, % by weight of allyl methacrylatec₃.

The sum of the components c₁ to c₃ is 100% by weight.

The monomers used include those stated for the second stage. Themonomers of the third stage need not be identical to those of the secondstage. Preferably, however, the same monomers as in the second stage areused. However, the third stage differs from the second stage in thequantitative composition of the monomer mixture. According to theinvention, the amount of vinylaromatic monomers c₁ is from 1.3 to 1.9times the amount of vinylaromatic monomers b₁.

The glass transition temperature of stage C is as a rule less than 0°C., preferably from -10° to -40° C.

In the fourth stage D, a monomer mixture is polymerized to an outer hardfourth stage D in the presence of the already formed latex comprisingthe stages A to C. The amount of this stage--also referred to as theouter hard phase--is from 15 to 30, preferably from 18 to 26, % byweight, based on the total amount of the emulsion graft copolymer, andconsists of

from 85 to 96, preferably from 90 to 96, % by weight of methylmethacrylate d₁,

from 3.8 to 10, preferably from 4 to 8, % by weight of C₁ -C₈ -alkylacrylate d₂,

from 0 to 2, preferably from 0 to 1, % by weight of di- orpolyfunctional comonomers d₃ and

from 0.2 to 3, preferably from 0.5 to 3, % by weight of a regulator d₄,

where d₂ and d₃ are to be understood as meaning the same monomers asstated for the first stage. The monomers d₂ and d₃ may differ from a₂and a₄. Preferably, however, the same monomers are used. The sum of thecomponents d₁ to d₄ is 100% by weight.

Suitable regulators (d₄) are monofunctional alkyl mercaptans, such assec-butyl mercaptan, n-dodecyl mercaptan and tert-dodecyl mercaptan.n-Dodecyl mercaptan is particularly suitable.

The presence of a regulator is particularly important since it limitsthe chain length in the polymerization of the end phase. In this way,the molecular weight of the outer hard phase D can be adapted to that ofthe thermoplastic material, which is advantageous particularly whenblending the emulsion graft copolymers with thermoplastic materials.

The glass transition temperature of the fourth stage D is greater than50° C., preferably from 80° to 130° C.

The novel emulsion graft copolymers can be prepared in a manner knownper se by the emulsion graft polymerization method, which is alsoreferred to as the seed latex method, the latex particles remaining inthe emulsified state up to the end of the final stage.

The total content of particles in the four-stage end product isdetermined by the polymeric product in the seed latex. This means that,after the first polymerization stage, essentially no new particles areformed and the particle content remains roughly constant.

By using emulsifiers, it is possible to establish the particle size ofboth the first stage and the latex particles in the subsequent stages,the type and concentration of emulsifier being critical.

Ionic and nonionic emulsifiers may be used.

Suitable emulsifiers are, for example, sodium dioctylsulfosuccinate,sodium laurylsulfate, sodium dodecylbenzenesulfonate,alkylphenoxypolyethylenesulfonates and salts of long-chain carboxylicand sulfonic acids.

Examples of suitable nonionic emulsifiers are fatty alcohol polyglycolethers, alkylaryl polyglycol ethers, fatty acid monoethanolamides andethoxylated fatty amides and fatty amines.

The total amount of emulsifier is preferably from 0.05 to 5% by weight,based on the total weight of the emulsion graft copolymer.

The diameter of the final emulsion polymer particles is brought in amanner known per se to 100-400 nm, for example 250-400 nm, preferably300-350 nm. In a further preferred embodiment, said diameter is from 150to 300 nm.

Ammonium and alkali metal peroxodisulfates, such as potassiumperoxodisulfate, and initiator combination systems, such as sodiumpersulfate, sodium hydrosulfite, potassium persulfate, sodiumformaldehyde sulfoxylate and potassium peroxodisulfate and sodiumdithionite/iron(II) sulfate, may be used as polymerization initiators,and the polymerization temperature may be from 50° to 100° C. in thecase of the ammonium and alkali metal peroxodisulfates to be thermallyactivated and may be lower, for example from 20° to 50° C., in the caseof the initiator combinations which are effective redox systems.

The total amount of initiator is preferably from 0.02 to 0.5% by weight,based on the prepared emulsion polymer.

In addition to the molecular weight regulator, which is used in thepolymerization of the outer hard stage (D), the polymerization of thefirst and other stages can also be carried out in the presence of aregulator. The total amount of regulator is in general from 0.01 to 1%by weight, based on the total weight of the polymer.

In general, it is advisable to keep the pH constant, and the presence ofa buffer is therefore advantageous. Salts of phosphoric acid, forexample mixtures of potassium dihydrogen phosphate and disodium hydrogenphosphate, are usual; ammoniacal solutions may also be used.

According to the novel process, the component C is always in excessrelative to the component B, ie. the amount of monomer mixture C isalways greater than that of monomer mixture B. According to the novelprocess, it is also necessary to ensure that the ratio of the amounts ofvinylaromatic monomers c₁ to b₁ is from 1.3:1 to 2.0:1, in particular1.4:1 to 1.9:1. This is preferably done by first polymerizing theelastomeric stage B onto the core A by feeding in the correspondingmonomer mixture B and then polymerizing a further elastomeric shell Conto the elastomeric shell B by feeding in the monomer mixture C.

The polymerization can be carried out at from 70° to 100° C., atemperature of from 85° to 95° C. being preferred. The polymerizationtimes increase at below 70° C. and the polymerization has to be carriedout under pressure at above 100° C.

In other respects, the novel emulsion graft copolymer is prepared byinitially taking an aqueous mixture consisting of monomers, crosslinkingagent, emulsifier, initiator, regulator and a buffer system in a reactorblanketed with nitrogen, establishing an inert atmosphere at roomtemperature while stirring and then bringing the mixture to thepolymerization temperature for from about 15 to 120 minutes. In thisway, the core latex A is first formed by emulsification andpolymerization of the core monomer mixture.

If necessary after a subsequent reaction time of from 15 to 120 minutes,stages C and D are produced by emulsion polymerization with feeding ofthe monomers in the presence of the already formed stage A. Then,possibly also after a subsequent reaction time, the outer hard phase Dis formed by emulsion polymerization of the corresponding monomers. Itmay be advantageous in each case to emulsify the monomers before thefeed while stirring in the aqueous medium.

The emulsion graft copolymer is isolated from the resulting latex in aknown manner by precipitation, filtration and subsequent drying. Forexample, aqueous solutions of inorganic salts, such as sodium chloride,sodium sulfate, magnesium sulfate and calcium chloride, aqueoussolutions of salts of formic acid, such as magnesium formate, calciumformate and zinc formate, aqueous solutions of inorganic acids, such assulfuric and phosphoric acid, and aqueous ammoniacal and amine solutionsand other aqueous alkaline solutions, for example of sodium hydroxideand potassium hydroxide, can be used for the precipitation.

The drying can be carried out, for example, by freeze drying, spraydrying, fluidized-bed drying and drying in a through-circulation oven.

The dried emulsion graft copolymer can then be processed in extruderswith acrylate resins to give impact-resistant thermoplastic materials.

Furthermore, coagulation and dewatering of the latex can be carried outdirectly in the extruder. For the preparation of the thermoplasticmaterials, the latex may also be mixed with the base resin directly inthe extruder.

The novel emulsion graft copolymers can be processed, either alone orwith blending with acrylate resins, in particular polymethylmethacrylate, to give impact-resistant moldings, such as sheets, filmsand coatings, not only by extrusion but also by other processingmethods, such as calendering and injection molding. Moreover, otheradditives, such as antioxidants, light stabilizers, heat stabilizers,lubricants, dyes and fillers may also be incorporated.

The novel emulsion graft copolymers are preferably used as additives forimproving the impact resistance of thermoplastic materials based onacrylate resins. Such acrylate resins are essentially composed of methylmethacrylate and up to 50% by weight of further comonomers. In the main,these are monofunctional monomers, such as C₁ -C₈ -alkyl acrylates andC₂ -C₈ -alkyl methacrylates. Polymers which contain from 80 to 99% byweight of methyl methacrylate, from 1 to 20% by weight of C₁ -C₈ -alkylacrylate, preferably methyl acrylate or ethyl acrylate, and from 0 to15% by weight of further comonomers are particularly preferred.

Preferred thermoplastic materials contain

I) from 10 to 60% by weight of emulsion graft copolymers as claimed inclaim 1 and

II) from 40 to 90% by weight of a polymer containing

II₁) from 80 to 99% by weight of methyl methacrylate,

II₂) from 1 to 20% by weight of C₁ -C₈ -alkyl acrylate and

II₃) from 0 to 15% by weight of further comonomers and

III) from 0 to 20% by weight of additives or processing assistants or ofmixtures thereof.

Thermoplastic materials which contain from 30 to 45% by weight ofemulsion graft copolymers and from 55 to 70% by weight of polymers IIare particularly preferred.

In addition to a toughness/rigidity ratio which is improved comparedwith the prior art, moldings of the novel thermoplastic materialsexhibit a substantially smaller tendency to form shell-like indentationson sawing; injection molded parts have a smooth residual surface afterremoval of the sprues. Compared with shaped articles of knownthermoplastic materials, the yellow tinge at the edges of shapedarticles of the novel materials is substantially reduced. The sameapplies to the haze.

EXAMPLES

The following abbreviations are used:

    ______________________________________                                        MMA              methyl methacrylate                                          MA               methyl acrylate                                              ALMA             allyl methacrylate                                           BA               butyl acrylate                                               S                styrene                                                      n-DCM            n-dodecyl mercaptan                                          ______________________________________                                    

Preparation of the emulsion graft copolymers

Comparative Example A

96 parts by weight of water, 0.34 part by weight of sodiumdioctylsulfosuccinate and 0.05 part by weight of potassiumperoxodisulfate were initially taken, together with 19 parts by weightof MMA, 0.85 part by weight of MA and 0.09 part by weight of ALMA, in apressure-resistant reactor having a volume of 800 l are were polymerizedat 90° C. for 30 minutes. A core A was obtained in this manner.

Thereafter, 72 parts by weight of BA, 15.8 parts by weight of styreneand 1.8 parts by weight of ALMA, together with 0.11 part by weight ofpotassium peroxodisulfate, were added to this emulsion andpolymerization was carried out at 90° C. for 1 hour. Stage B formed.

Thereafter, 27 parts by weight of MMA, 1.3 parts by weight of MA, 0.18part by weight of nDCM and 0.02 part by weight of potassiumperoxodisulfate were added and polymerization was carried out at 90° C.for 1 hour until a conversion of 98% had been reached. A further shell Cresulted.

Example 1

A core A was prepared similarly to Comparative Example 1.

Thereafter, 72 parts by weight of BA, 15.8 parts by weight of styrene,1.8 parts by weight of ALMA and 0.11 part by weight of potassiumperoxodisulfate were added in the course of 2 hours to the resultingemulsion at 90° C. The monomer mixture was added by a procedure in whichfirst a total of 46% by weight of the total monomer amount of stages Band C was polymerized for 1 hour, this feed containing 35% by weight ofthe total styrene. Stage B thus formed.

54% by weight of the total monomer amount B and C were then polymerizedat 90° C. for 1 hour, the feed now containing 65% by weight of the totalstyrene. The styrene-richer stage C resulted.

Finally, 27 parts by weight of MMA, 1.3 parts by weight of MA, 0.18 partby weight of nDCM and 0.02 part by weight of potassium peroxodisulfatewere also added while polymerization was carried out at 90° C. for 1hour until a conversion of at least 98% had been reached. It waspossible to prepare shell D in this manner.

Example 2

An emulsion graft copolymer having shells A, B, C and D was preparedsimilarly to novel Example 1, except that the feed of the monomermixtures B and C was such that first a total of 49% by weight of thetotal monomer amount of stages B and C was polymerized for 1 hour andthis first feed contained 40% by weight of the total styrene. 51% byweight of the total monomer amount of stages B and C were thenpolymerized, this second feed now containing 60% by weight of the totalstyrene.

Comparative Example B

An emulsion graft copolymer having shells A, B, C and D was preparedsimilarly to novel Example 2, except that the feed of the monomermixtures B and C was changed so that the first feed contained 47% byweight of the total monomer amount B and C and 32% by weight of thetotal styrene. Accordingly, the second feed contained 53% by weight ofthe total monomer amount B and C and 68% by weight of the total styrene.

Comparative Example C

An emulsion graft copolymer having shells A, B, C and D was preparedsimilarly to novel Example 2, except that the feed of the monomermixtures B and C was changed so that the first feed contained 50% byweight of the total monomer amount B and C and 45% by weight of thetotal styrene. Accordingly, the second feed contained likewise 50% byweight of the total monomer amount B and C and 55% by weight of thetotal styrene.

Testing of performance characteristics

The following properties were determined:

impact strength a_(N) according to DIN 53 453

light transmittance τ according to DIN 5036 (6 mm)

haze according to DIN 5036 (6 mm)

yellowness index YI according to ASTM-D 1925

modulus of elasticity according to DIN 53457-3

formation of shell-like indentations

splintering.

The impact strength a_(N) was tested by the Charpy method. In this testmethod, a standardized test specimen resting on two supports isdestroyed or damaged by an impact aimed at its center. The energy (kJ)required for the destruction or damage is based on the criticalcross-section (m²) and defined as impact strength a_(N) (dimensionkJ/m²).

The light transmittance τ (dimension %) is defined as the ratio of thelight transmitted by a sample to the intensity of the incident light. Itwas determined as a function of the wavelength in a range from 400 to900 nm using a 6 mm thick test specimen.

Transparent plastics for which optical clarity is essential for use areinvestigated to determine their haze. The haze (dimension %) is definedas that part of the light transmitted by a material sample whch deviatesfrom the direction of the light ray incident on the sample, owing toscattering which takes place on the material. The haze was determinedusing a 6 mm thick test specimen.

The yellowness index YI is measured on the basis of color coordinateswhich are calculated from the transmission spectrum according to DIN5036. The illuminant D65 and the wide-field standard observer are usedas a basis.

The modulus of elasticity was determined by applying a tension to astandardized test specimen at a standardized draw-off speed according toDIN 53457-3. The modulus of elasticity was determined from the slope ofthe tangent at the origin of the tension-stress curve.

The composition of the emulsion graft copolymers and the results of theindividual tests of the performance characteristics are shown inTable 1. The compositions were chosen so that the content of elastomericstages is the same in all examples. The results of the tests show thatthe novel emulsion graft copolymers are superior to the prior artemulsion polymers in their mechanical, thermal and optical properties.

                                      TABLE                                       __________________________________________________________________________    Example   VA       1           2                                              Stage     .sup.1 H.sup.1)                                                                  .sup.2 S.sup.1)                                                                  .sup.3 S                                                                         A.sub.1                                                                          B.sub.1                                                                          C.sub.1                                                                          D.sub.1                                                                          A.sub.2                                                                          B.sub.2                                                                          C.sub.2                                                                          D.sub.2                               __________________________________________________________________________    Component                                                                      % by wt.!:                                                                   Stage     14 65 21 14 30 35 21 14 31.2                                                                             33.5                                                                             21                                    MMA       95.3  95.0                                                                             95.3     95.0                                                                             95.3     95.0                                  MA        4.2   4.5                                                                              4.2      4.2                                                                              4.2      4.5                                   ALMA      0.5                                                                              2.0   0.5                                                                              1.0                                                                              1.0   0.5                                                                              1.0                                                                              1.0                                      BA           80.4     85.1                                                                             77.2     84.3                                                                             78.4                                     S            17.6     12.9                                                                             21.8     14.7                                                                             20.6                                     nDCM            0.5         0.5         0.5                                   Proportion of the                                                                       -        54          51                                             second feed (Shell                                                            C)                                                                             % by wt.!                                                                    Ratio of the styrene                                                                    -        1.86        1.5                                            amounts                                                                       (C.sub.1 :b.sub.1)                                                            Properties:                                                                   τ     90.8     92.0        92.0                                           Haze      2.0      1.8         1.7                                            YI        1.1      0.9         0.8                                            YI (edge) ++       0           -                                              a.sub.N  kJ/m.sup.2 !                                                                   76.2     86.7        89.5                                           Modulus of                                                                              1660     1800        1823                                           elasticity  Nm/mm.sup.2 !                                                     Formation of shell-                                                                     12       5           3                                              like indentations                                                             Number of defects/m                                                           Defect size  mm!                                                                        >1       <0.5        ≦0.5                                    Tendency to                                                                             ++       -           -                                              splinter                                                                      __________________________________________________________________________    Example    VB              VC                                                 Stage      A.sub.3                                                                           B'.sub.3                                                                          C'.sub.3                                                                          D.sub.3                                                                           A.sub.4                                                                           B'.sub.4                                                                          C'.sub.4                                                                          D.sub.4                                __________________________________________________________________________    Component                                                                      % by wt.!:                                                                   Stage      14  30  35  21  14  31  33  21                                     MMA        95.3        95.0                                                                              95.3        95.0                                   MA         4.2         4.5 4.2         4.5                                    ALMA       0.5 1.0 1.0     0.5 1.0 1.0                                        BA             87.5                                                                              75.8        82.7                                                                              79.B                                       S              11.5                                                                              23.2        16.3                                                                              19.2                                       nDCM                   0.5             0.5                                    Proportion of the                                                                        53              50                                                 second feed (Shell                                                            C)                                                                             % by wt.!                                                                    Ratio of the                                                                             2.1             1.2                                                styrene amounts                                                               (C.sub.1 :b.sub.1)                                                            Properties:                                                                   τ      91.7            91.7                                               Haze       2.2             2.0                                                YI         1.4             1.0                                                YI (edge)  +               +                                                  a.sub.N  kJ/m.sup.2 !                                                                    79.6            83.0                                               Modulus of 1760            1785                                               elasticity  Nm/mm.sup.2 !                                                     Formation of shell-                                                                      10              13                                                 like indentations                                                             Number of defects/m                                                           Defect size  mm!                                                                         ≦0.5     ≧0.5                                        Tendency to                                                                              0               +                                                  splinter                                                                      __________________________________________________________________________     .sup.1) H: hard (nonelastomeric) stage                                        S: soft (elastomeric) stage                                                   .sup.2) ++: very substantial, +: substantial, 0: slight, -: not present  

We claim:
 1. An emulsion graft copolymer composed ofA) from 5 to 18% byweight of a first stage comprisinga₁) from 85 to 99% by weight of methylmethacrylate, a₂) from 1 to 15% by weight of C₁ -C₈ -alkyl acrylate, a₃)from 0 to 2% by weight of allyl methacrylate and a₄) from 0 to 3% byweight of further di- or polyfunctional comonomers, B) from 25 to 35% byweight of a second stage comprisingb₁) from 10 to 25% by weight ofvinylaromatic monomers, b₂) from 75 to 90% by weight of C₁ -C₂₀ -alkylacrylate and b₃) from 0 to 3% by weight of allyl methacrylate, C) from30 to 40% by weight of a third stage comprisingc₁) from 15 to 27% byweight of vinylaromatic monomers, c₂) from 73 to 85% by weight of C₁-C₂₀ -alkyl acrylate and c₃) from 0 to 3% by weight of allylmethacrylate and D) from 15 to 30% by weight of a fourth stagecomprisingd₁) from 85 to 96% by weight of methyl methacrylate, d₂) from3.8 to 10% by weight of C₁ -C₈ -alkyl acrylate, d₃) from 0 to 2% byweight of further di- or polyfunctional comonomers and d₄) from 0.2 to3% by weight of a regulator,with the proviso that the ratio of theamounts of vinylaromatic monomers c₁ :b₁ is from 1.3:1 to 1.9:1 and thesum of the components A to D is 100% by weight.
 2. A process for thepreparation of an emulsion graft copolymer, whereinA) from 5 to 18% byweight of a monomer mixture comprisinga₁) from 85 to 99% by weight ofmethyl methacrylate, a₂) from 1 to 15% by weight of C₁ -C₈ -alkylacrylate, a₃) from 0 to 2% by weight of allyl methacrylate and a₄) from0 to 3% by weight of further di- or polyfunctional comonomers, B) from25 to 35% by weight of a monomer mixture comprisingb₁) from 10 to 25% byweight of vinylaromatic monomers, b₂) from 75 to 90% by weight of C₁-C₂₀ -alkyl acrylate and b₃) from 0 to 3% by weight of allylmethacrylate, C) from 30 to 40% by weight of a monomer mixturecomprisingc₁) from 15 to 27% by weight of vinylaromatic monomers, c₂)from 73 to 85% by weight of C₁ -C₂₀ -alkyl acrylate and c₃) from 0 to 3%by weight of allyl methacrylate and D) from 15 to 30% by weight of amonomer mixture comprisingd₁) from 85 to 96% by weight of methylmethacrylate, d₂) from 3.8 to 10% by weight of C₁ -C₈ -alkyl acrylate,d₃) from 0 to 2% by weight of further di- or polyfunctional comonomersand d₄) from 0.2 to 3% by weight of a regulator,are polymerized insuccession, wherein the sum of the components A to D is 100% by weightand the component C is always in excess relative to the component B andthe ratio of the amounts of vinylaromatic monomers c₁ :b₁ is from 1.3 to2.0:1.
 3. A process as claimed in claim 2, wherein the ratio of theamounts of vinylaromatic monomers c₁ to b₁ is from 1.4:1 to 1.9:1.
 4. Athermoplastic material containing from 10 to 60% by weight of anemulsion graft copolymer as claimed in claim
 1. 5. A thermoplasticmaterial as claimed in claim 4, containingI) from 10 to 60% by weight ofan emulsion graft copolymer as claimed in claim 1 and II) from 40 to 90%by weight of a polymer containingII₁) from 80 to 99% by weight of methylmethacrylate, II₂) from 1 to 20% by weight of C₁ -C₈ -alkyl acrylate andII₃) from 0 to 20% by weight of further comonomers and III) from 0 to20% by weight of additives or processing assistants or of mixturesthereof.
 6. A molding, film or coating, containing a thermoplatsicmaterial as claimed in claim
 4. 7. A thermoplastic material withincreased impact resistance containing an emulsion graft copolymer asclaimed in claim 1.